Unlike most consonant declension nouns, the dative plural ἀσπίσιν does not undergo compensatory lengthening, retaining a short iota despite losing the delta when the ending beginning in sigma is added. Why is that?

It seems compensatory lengthening in Dative Plural occurs only in stems ending in -ντ-,where -ντ- drops and preceding vowel ο becomes ου. In dental stems such as ἀσπιδ-,and nasals like δαιμον- where ν drops (technically not; see Smyth 250b N. about it) without lengthening of preceding vowel, there is none. If the vowel is long to begin with, then of courseit stays long. See table with examples in Smyth 257.